Magnetic resonance imaging is a known technology able to create images of the interior of an object to be examined. To do this, it employs the dependence of the precision frequencies (Larmor frequencies) of excited spins on the magnetic field strength of the prevailing magnetic field of the magnetic resonance device for local resolution. Hereby, the prevailing magnetic field comprises the main magnetic field of the magnetic resonance device and applied gradient magnetic fields. Conventional methods for the reconstruction of image data records from magnetic resonance signals require a homogeneous main magnetic field and strictly linear gradient magnetic fields.
Due to the dependence of Larmor frequencies on the prevailing magnetic field, inhomogeneities of the main magnetic field result in geometric distortions along the frequency encoding direction (read-out direction) in the image data records obtained from the magnetic resonance signals. Hereby, the distortions are proportional to local deviations of the main magnetic field and vice versa proportional to the strength of the frequency encoding gradient.
In practice, it is not possible completely to avoid inhomogeneities of the main magnetic field of this kind. Nevertheless, within a measuring volume of a magnetic resonance device, the deviations of the main magnetic field, i.e. the inhomogeneity, should still be less than 3 ppm (ppm: “parts per million”).
Inhomogeneities of main magnetic fields in magnetic resonance devices are for example design-induced, i.e. they are, for example, dependent upon the design and winding geometry of the main field magnets, the screening and shim devices provided. Inhomogeneities of the main magnetic field induced in this way are static, i.e. they remain substantially temporally constant.
It is usual to employ a measuring phantom to determine static inhomogeneities of the main magnetic field. For this, the measuring phantom is used to measure the actual magnetic field at several measuring points on a surface of a conductor-free volume. The values measured at the measuring points can be used in a known way to determine the main magnetic field at every point within the volume. Hereby, the accuracy of the determination of the main magnetic field depends, on the one hand, on the measuring accuracy of the measuring phantom and, on the other, on the accuracy of the algorithm for the determination of the main magnetic field from the measuring points. The inhomogeneity is then obtained, for example, from the relative deviation of the measured main magnetic field from a setpoint value.
Further causes of inhomogeneities of a magnetic field in a magnetic resonance unit are, for example, susceptibility changes caused by an object to be examined introduced into the magnetic resonance unit, dynamic interference from eddy currents or artifacts such as “Chemical Shift”, flow artifacts or movements of the object to be examined. Inhomogeneities caused in this way depend upon the situation in question, for example the type of the examination and the object to be examined.
Any kind of distortion in image data records is undesirable, in particular in medical image data records, since they could falsify, or at least complicate, a diagnosis. Due to the various possible causes and types of distortion there are already various methods which utilize knowledge of the inhomogeneities of the main magnetic field to correct distortion in image data records.
For example, WO 95/30908 A1 discloses a method in which a generalized Fresnel transform (GFT reconstruction) is performed in the read-out direction. The GFT reconstruction takes into account a known position dependence of the main magnetic field in the read-out direction in order to be able to correct distortion and intensity errors during the transformation from the measured data space (k-space) into the position space.
In addition, the most accurate possible determination of local deviations of the main magnetic field of a magnetic resonance device is important for an optimum improvement of the main magnetic field homogeneity, for example in the form of shim elements. The improvement of the main magnetic field homogeneity is in turn of great importance for the improvement of applications such as measurements with a large metering field or spectroscopic examinations.